In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a body vessel such as an arterial or vascular passageway and which are subsequently expandable into contact with walls of the passageway. These devices, among others, include stents, such as those shown by U.S. Pat. No. 5,540,712 and blood clot filters such as those shown by U.S. Pat. No. 5,669,933 which expand and are held in position by engagement with the inner wall of a vessel. It has been found to be advantageous to form such devices of a thermal shape memory material having a first, relatively pliable low temperature condition and a second, relatively rigid high-temperature condition. By forming such devices of temperature responsive material, the device in a flexible and reduced stress state may be compressed to fit within the bore of a delivery catheter when exposed to a temperature below a predetermined transition temperature, but at temperatures at or above the transition temperature, the device expands and becomes relatively rigid.
Known self expanding medical devices have been formed of Nitinol, an alloy of titanium and nickel which provides the device with a thermal memory. The unique characteristic of this alloy is its thermally triggered shape memory, which allows a device constructed of the alloy to be cooled below a temperature transformation level to a martensitic state and thereby softened for loading into a catheter in a relatively compressed and elongated state, and to regain the memorized shape in an austenitic state when warmed to a selected temperature, above the temperature transformation level, such as human body temperature. The two interchangeable shapes are possible because of the two distinct microcrystalline structures that are interchangeable with a small variation in temperature. The temperature at which the device assumes its first configuration may be varied within wide limits by changing the composition of the alloy. Thus, while for human use the alloy may be focused on a transition temperature range close to 98.6.degree. F., the alloy readily may be modified for use in animals with different body temperatures.
In recent years advances have been made in the treatment of blood vessel stenosis or occlusion by plaque, thrombi, embolic, or other deposits which adversely reduce or block the flow of blood through a vessel. Balloon angioplasty or similar transluminal treatments have become common for some blood vessel lesions, but for all such procedures, plaque and emboli dislodged during the procedure are free to flow within the lumen of the vessel and possibly cause substantial injury to a patient.
In an attempt to contain and remove emboli and other debris, balloon angioplasty coupled with irrigation and aspiration has been performed as illustrated by U.S. Pat. No. 5,883,644 and International Publication No. WO 98/39046 to Zadno-Azizi et al. This procedure requires complete vessel occlusion cutting off all blood flow which imposes severe time constraints on the procedure. Additionally, the balloons involved in the procedure are affixed to elongate guidewires or small elongate catheters which extend for a substantial distance through blood vessels to the location of the stenosis or occlusion, and it is practically impossible to prevent some back and forth longitudinal motion of these elongate elements within a vessel during a procedure. This movement of the guidewire or catheter to which a balloon is attached causes the balloon to move back and forth and abrade emboli from the vessel wall downstream of the balloon containment area.
Angioplasty is often not a preferred treatment for lesions in the carotid artery because dislodged plaque can enter arterial vessels of the brain causing brain damage or even death. As indicated by U.S. Pat. No. 5,879,367 to Kaganov et al., carotid endarterectomy is a surgical procedure used to remove a lesion in the carotid artery, but this procedure also involves substantial risk of dislodged embolic material.
In an attempt to contain dislodged emboli during a procedure to clear blood vessel stenosis or occlusion, a variety of distal filters have been devised such as those shown by U.S. Pat. No. 5,814,064 and International Publication Nos. WO 98/38920 and WO 98/39053 to Daniel et al. as well as U.S. Pat. No. 5,827,324 to Cassell et al., U.S. Pat. No. 5,846,260 to Maahs and U.S. Pat. No. 5,876,367 to Kaganov et al. These filters are secured to the distal portion of a guidewire or catheter and are deployed distally from the stenosis or occlusion to capture embolic material. Once the distal filter is positioned and expanded into contact with the wall of the blood vessel, an angioplasty balloon, a stent, or other devices are introduced over the proximal end of the guidewire or catheter to which the filter is attached and moved into position in the area of the occlusion or stenosis spaced proximally from the filter.
Known guidewire or catheter attached distal filters have been subject to a number of disadvantages. First, since the elongate catheter or guidewire to which the filter is attached is used to guide over the wire devices during a subsequent procedure, it is extremely difficult if not impossible to prevent longitudinal movement of the wire or catheter after the filter has been deployed. This causes the filter to move back and forth within the vessel with resultant abrasion by the filter of the vessel wall, and such abrasion not only causes trauma to the vessel wall but also operates to dislodge debris which is free to flow distally of the filter. Thus filter movement after the filter is deployed somewhat defeats the purpose of the filter. Also, it is often desirable during a procedure to exchange guidewires, and such an exchange is not possible with an attached filter.
Finally the retrieval of known distal filters while retaining captured embolic material has proven to be problematic. Many cone shaped filters with wide, upstream proximal open ends tend to eject captured embolic material through the open end as the filter is collapsed. Also, many distal filters are formed by a mesh material which is expanded by a filter frame, and when the frame closes to collapse the filter for withdrawal through a catheter, the mesh folds creating outwardly projecting pleats. These pleats snag on the withdrawal catheter making retrieval of the filter difficult and often causing the filter to spill captured embolic material.